The present invention relates to a method for producing hydrogen by microorganisms. More specifically, the present invention relates to a method for producing hydrogen efficiently with a microalga having a photosynthetic ability and a bacterium having a photosynthetic ability, utilizing solar energy.
In recent years, global warming has been a main factor that causes abnormal weather worldwide and breaks the global ecosystem, so that prevention of global warming is a worldwide issue that should be addressed urgently. It has turned out that global warming is caused mainly by a large consumption of fossil fuel including oil and coal and the resulting warming gases such as carbon dioxide (CO2). In this context, there is a strong demand for research in new technologies for suppressing the release of global warming gases, creation of energy sources, such as hydrogen, utilizing biofunctions, and study of immobilization and degradation of global warming gases.
Among these, hydrogen energy is advantageous in that it can be converted to electrical energy with high efficiency in the form of a fuel cell, the amount of the generated heat is 3 or 4 times that generated by oil, only water is generated after combustion so that there is no fear of environmental pollution, and that water, which is the raw material for hydrogen energy, is abundant.
Various studies for production of hydrogen energy utilizing biofunctions have been conducted. For example, Japanese Laid-Open Patent Publication (Tokkai) No. 58-60992 discloses a method for producing hydrogen by culturing a green alga having a photosynthetic ability and a hydrogen producing ability under light and aerobic conditions and degrading the photosynthesized and accumulated substances under dark and slightly aerobic conditions to produce hydrogen. Furthermore, Farmacia vol. 26, 419-422 (1990) reports that among green algae, the Chlamydomonas sp. strain MGA161 has a high hydrogen producing ability. Biosci. Biotech. Biochem., vol. 56, 751-754 (1992) reports that hydrogen can be produced efficiently by degrading the photosynthetic products by Chlamydomonas sp. strain MGA161 under dark and anaerobic conditions and allowing a bacterium having a photosynthetic ability (hereinafter, referred to as xe2x80x9cphotosynthetic bacteriumxe2x80x9d) to act on the degraded products. This article describes that the photosynthetic bacterium produces hydrogen, using ethanol, acetic acid, formic acid, lactic acid, etc., which are products of fermentative degradation of the photosynthetic products by the green alga, as electron donors. On the other hand, the article also reports that hydrogen production cannot be achieved simply with ethanol or acetic acid. Thus, the mechanism for the production of hydrogen is not clear, and therefore optimum conditions have not been established yet.
On the other hand, hydrogen production with a bacterium is not performed on an industrial scale, because of its culture apparatus. For example, cultivation of a photosynthetic green alga and a photosynthetic bacterium requires light, but light cannot reach the deep portion of a culture tank. Therefore, the culture tank should be shallow, which causes a problem with regard to the culture site that a large floor area is required for mass cultivation. In addition, a shallow culture causes a problem with regard to the culture conditions as follows. Since the culture tank is shallow, dissolution of carbon dioxide in the culture medium is insufficient, so that the cultivation is inefficient. Moreover, continuous cultivation is also difficult.
In order to solve these problems, methods for culturing with stirring while irradiating the culture with artificial light have been examined. However, there are some problems with regard to insufficiency of light that can reach the inside, irradiation time of cells with light, and irradiation quantity. Furthermore, the cost of artificial light is problematic, so that problems for mass production of hydrogen remain unsolved.
Therefore, in systems for hydrogen production with a microalga and a photosynthetic bacterium, there is a need to establish efficient cultivation of a photosynthetic microalga and optimum conditions for hydrogen production by a photosynthetic bacterium that uses products by fermentative degradation of photosynthetic products as substrates.
It is an object of the present invention to solve the above problems, and provide efficient cultivation of a photosynthetic microalga and optimum conditions for hydrogen production by a photosynthetic bacterium that uses products by fermentative degradation of photosynthetic products as substrates, as well as a method for mass production of hydrogen by microorganisms.
The present invention provides a method for producing hydrogen comprising preparing photosynthetic products by culturing a microalga having a photosynthetic ability under light and aerobic conditions; preparing a fermentation solution of the photosynthetic products by culturing the microalga under dark and anaerobic conditions; and allowing a bacterium having a photosynthetic ability to act on the fermentation solution under light and anaerobic conditions, wherein the process of culturing the microalga under light and aerobic conditions and/or the process of allowing the bacterium having a photosynthetic ability to act on the fermentation solution under light and anaerobic conditions are performed with a transparent tower-type air-lift culture tank.
In a preferable embodiment, the tower-type air-lift culture tank is of a dual cylindrical structure.
In another preferable embodiment, the tower-type air-lift culture tank is of a structure in which a helical flow is generated.
In still another preferable embodiment, the subsequent processes are performed continuously: preparing photosynthetic products by culturing the microalga having a photosynthetic ability under light and aerobic conditions; preparing a fermentation solution of the photosynthetic products by culturing the microalga under dark and anaerobic conditions; and allowing a bacterium having a photosynthetic ability to act on the fermentation solution for hydrogen production under light and anaerobic conditions.
In yet another preferable embodiment, in the process of allowing the bacterium having a photosynthetic ability to act on the fermentation solution under light and anaerobic conditions, a lactic acid content is regulated to at least 0.3 mM.
According to another aspect of the present invention, a method for producing hydrogen includes preparing photosynthetic products by culturing a microalga having a photosynthetic ability under light and aerobic conditions; preparing a fermentation solution of the photosynthetic products by culturing the microalga under dark and anaerobic conditions; and allowing a bacterium having a photosynthetic ability to act on the fermentation solution under light and anaerobic conditions, wherein a lactic acid content in the fermentation solution under the dark and anaerobic conditions is regulated to at least 0.3 mM.